1. Field of the Invention
The present invention relates to a mobile communication network, and in particular to a cell selecting system and a method thereof in a radio communication network.
2. Description of the Background
In general, a radio communication system such as a GPRS (general packet radio service) and a GSM (global system for mobile communications), etc. guarantees continuous Internet contact at a high data transmission speed to mobile terminals and computer users.
In a channel usage method of the radio communication system, a channel is used after mapping a logical channel to a physical channel such as a RF (radio frequency) channel, a TDMA (time division multiple access) frame and a time slot, etc. There are a TCH (traffic channel), a control channel, etc. in the logical channel. The TCH is for transmitting user data, and the control channel is for transmitting control signal information.
A broadcast control channel (BCCH) is one of the known control channels. The BCCH is a point to multi-point forward single direction channel and is used for information notification to plural mobile terminals. All mobile terminals are required to receive the BCCHs from cell base stations before a network contact can be made.
FIG. 1 illustrates a construction of a forward BCCH generated by a cell base station and used in a general radio communication system.
As depicted in FIG. 1, the BCCH consists of frames, and each frame consists of eight (8) time slots. The BCCH includes a FCCH (frequency correction channel) frame 10 and a SCH (synchronization channel) frame 12. The FCCH frame 10 and the SCH frame 12 abut each other. The FCCH frame generally carries information to perform frequency connection of a mobile terminal. The SCH frame 12 provides the mobile terminal with information it needs for its initial synchronization with a network.
As shown, the FCCH frame and the SCH frame respectively always have a FCCH bit pattern and a SCH bit pattern at a first time slot (time slot 0). The rest of the time slots (time slots 1˜7) of the FCCH frame and the SCH frame are not used and have pre-determined bit patterns known as dummy burst.
Although the not-used time slots exist in each frame of the BCCH, the output power of the BCCH channel is always maintained uniformly regardless of a time slot in order to make it possible for a mobile terminal to select any time slot during a reception power measurement operation of the BCCH channel monitoring process.
A cell selecting method of a mobile terminal using the above-discussed general BCCH will be described. In general, when a mobile terminal is powered-on, the mobile terminal has to select a usable cell. Then the mobile terminal searches for cells around the selected cell periodically or sequentially, so that it can select a new cell as needed (e.g., as the mobile terminal is moved or in a handover process).
FIG. 2 is a flow chart illustrating a general cell selecting method of a conventional mobile terminal using the general BCCH of FIG. 1. Referring to FIG. 2, in order for a mobile terminal to select a cell in the early stage, the mobile terminal monitors sequentially the BCCH of each of plural cells nearby as shown at step S11. Here, each cell base station transmits its BCCH. Then the mobile terminal measures the reception power of the BCCH of each of the monitored plural cells and identifies pertinent cells having the reception power not less than a certain size as shown at step S13.
Afterward, the mobile terminal once again monitors the BCCHs of the identified cells in the order of the reception power level size as shown at step S15. Then it accesses, stores and demodulates sequentially (in the order of the reception power level size) a FCCH bit pattern and then a SCH bit pattern from each of the monitored BCCHs as shown at step S17. The mobile terminal obtains timing information from the SCH bit pattern demodulation.
Then the mobile terminal selects an appropriate cell from the identified cells based on their reception power measurements (i.e., the strength of signals received from the cell base stations) and restores SI (system information) from the timing information obtained from the SCH bit pattern demodulation of the BCCH of the selected appropriate cell as shown at step S19.
For a reselection of a cell or a selection of a new cell, the above-described cell selection method of FIG. 2 is repeated.
In the cell selection method, however, after the mobile terminal measures the BCCH reception power of cells, there is no guarantee that the mobile terminal accurately accesses a first time slot (time slot 0) of the FCCH (and/or SCH) frame of the BCCH of a pertinent cell. Since the FCCH and SCH bit patterns are stored only at time slot 0 of the corresponding frame, there is a high probability that the mobile terminal will access other time slots (time slots 1˜7) first when it is time to access the FCCH/SCH bit patterns.
Accordingly, in the above-described cell selecting method, the mobile terminal first must measure the reception power of the BCCHs of multiple cells, and then must re-monitor the BCCHs of certain pertinent cells in the order of the size of reception power to locate and detect a first time slot having a FCCH bit pattern and a first time slot having a SCH bit pattern, of the frames of each pertinent cell. Thus, certain cells are monitored repeatedly in time by the mobile terminal.
Because the same cells are monitored repeatedly (i.e., to measure the reception power of BCCHs and to access and demodulate FCCH bit patterns and SCH bit patterns of the BCCHs), a longer cell synchronization time is consumed, and accordingly a cell selecting time is prolonged undesirably.
In addition, in the above-described cell selecting method, by monitoring the BCCHs of the same cells repeatedly, a processing load of the mobile terminal is increased greatly.